The three basic tasks of a mass spectrometer are to generate ionic, gaseous versions of analytes in a source, transfer the analyte ions from the source through several differential pumping regions, and finally measure the abundance and mass-to-charge ratios (m/z's) of the analyte ions or product ions derived therefrom. The movement of ions from one location to another in the instrument is controlled primarily through the application of oscillatory and/or static voltages to the various ion transfer optic devices (e.g., radio-frequency multipoles, stacked-ring ion guides, and electrostatic lenses) to establish electric fields that radially confine the ions to a central ion path and urge the ions along a longitudinal trajectory from regions of higher to lower potential energy. As is well known in the mass spectrometry art, these ion transfer optic devices must be kept clean and free from particles and debris, which can cause degradation of instrument performance, by a process commonly referred to as “charging”, leading to loss of sensitivity. The mechanism of degradation of instrument performance is thought to occur in two steps. Contamination is introduced onto an optical element of the ion path in one of several ways, e.g., from the room environment or device mishandling when the instrument was open for service, or from the atmospheric ionization source while the instrument was under vacuum. When ions subsequently impinge on these non-conductive contaminants, their charge can dissipate only slowly. Over time, enough charge can accumulate to create a voltage potential leading to a significant aberration in the local electrical field, such that new ions are either deflected away from their intended path, or blocked completely. Such aberrations often have mass-dependent effects, whereby ions of different m/z's are affected disproportionately. Small particles like dust and fibers have high aspect ratios, such that large electric fields can be generated from small numbers of impinging ions, and so charging occurs fairly rapidly once they are exposed to ions. As instrument developers strive to increase instrument sensitivity, atmospheric ionization source orifices grow ever larger, increasing the probability of contaminants entering the system and potentially causing contamination and charging to occur faster, requiring the instrument to be serviced more frequently.
If an instrument shows signs of sensitivity decrease, the presence of charging is commonly diagnosed by the method of switching instrument polarity, i.e., by switching the polarity, e.g., from positive to negative, of analyte ions produced by the source and delivered through the ion transfer optic devices to the mass analyzer. In an illustrative example, decreased instrument sensitivity may be suspected when operating a mass spectrometer to analyze positive analyte ions. When the instrument is switched from positive, to negative, and back to positive polarity, the instrument sensitivity may be temporarily restored (due to the rapid neutralization of positively charged ion transfer optic surfaces by the impingement thereon of negative ions) and so monitoring ion abundance during this procedure will show a characteristic jump in intensity. This method can help to discern the presence of contamination somewhere in the instrument, but cannot be more precise.
Against this background, there is a continuing need for a method for identifying the specific location of the occurrence of charging within a mass spectrometer.